1. Field of the Invention
The present invention relates to a method for analyzing impurities, and specifically, relates to a method for analyzing impurities to analyze impurities present in a silicon wafer.
This application claims priority from Japanese Patent Application No. 2004-153713 filed on May 24, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, there have been demands for even greater degrees of flatness in silicon wafers. Therefore, there have also been demands for even greater degrees of flatness in a etching process. In this etching process for planarizing a silicon wafer, an alkaline etching solution is often used which is able to obtain an etched surface more excellent in flatness than that obtained by an acid etching solution. For example, a potassium hydroxide (KOH) solution or a sodium hydroxide (NaOH) solution is often used as the alkaline etching solution. However, a minute quantity of impurities are present in the KOH or NaOH solution, regardless of a concentration of the impurities. Examples of such impurities include Ni. This Ni has a tendency to be easily diffused into a silicon layer. Namely, when a silicon wafer is subjected to an alkaline etching, Ni diffuses from the silicon layer of the surface. As a result, the silicon wafer is contaminated by Ni.
When the silicon wafer is contaminated by Ni, the Ni is located at a particularly high concentration in a region from a surface of the silicon wafer to 1 μm in a depth direction. Ni concentration is lowered as a depth increases. Moreover, when a Ni-diffused silicon wafer is subjected to a heat treatment after the alkaline etching, the diffusion of Ni into bulk portions of the silicon wafer progresses even further.
When the bulk portions of the silicon wafer are contaminated by Ni, a problem arises in which for example, a gate oxide integrity in the device is degraded so that performance of the device is deteriorated. Accordingly, Ni contamination of the silicon wafer is a serious problem. Therefore, a method for analyzing impurities is required so as to confirm a degree of Ni contamination in the bulk portions of the silicon wafer.
Examples of the methods for analyzing impurities in the silicon wafer include total reflection x-ray fluorescence (TXRD), secondary ion mass spectrometry (SIMS), atomic absorption spectroscopy (AAS), and inductively coupled plasma mass spectrometry (ICP-MS).
A concrete example of a method for evaluating a degree of Ni contamination in a silicon wafer using the above analytical methods is a method for analyzing impurities in a surface of the wafer disclosed in Patent Document 1. In this Patent Document, a composition of a liquid mixture of hydrofluoric acid and nitric acid is adjusted in response to a distribution state of the impurities. The adjusted liquid mixture is supplied to the surface of the silicon wafer, and then after a predetermined time, the liquid mixture is recovered and concentrated, and analysis is then performed.
In a method for analyzing impurities in a silicon substrate and in a vapor phase thereof disclosed in Patent Document 2, a sample of a whole silicon wafer or a sample including a portion extending from a front surface to a rear surface of the silicon wafer which is cleaved, and a dissolving solution of a mixture of hydrofluoric acid, nitric acid, and sulfuric acid are set in a sealed reactor vessel such that the dissolving solution for silicon does not make contact with the sample. Then the solution is vaporized without the reactor vessel being heated or pressurized. As a result, the silicon wafer is dissolved by the vaporized solution. Subsequently, a dissolved liquid is recovered, and analysis of the impurities therein is performed.
Furthermore, in a method for evaluating a silicon wafer disclosed in Patent Document 3, observation of a surface state after etching and analysis of impurities are performed using an alkaline etching solution.
When using the method disclosed in Patent Document 1 alone, impurities in a surface layer of the silicon wafer can be analyzed, however impurities in a whole silicon bulk cannot be analyzed.
Moreover, in the method disclosed in Patent Document 2, great deal of time is required in order to dissolve the silicon wafer. In addition, a large quantity of acid is used in order to dissolve the silicon wafer so that a cost of chemical solutions used is expensive.
Furthermore, in the method disclosed in Patent Document 3, because impurities are observed visually, problems arise that a type of metal included in the impurities cannot be specified and a numerical value for an amount of contamination cannot be estimated.
(Patent Document 1) Japanese Unexamined Patent Application, First Publication No. H07-130808
(Patent Document 2) Japanese Unexamined Patent Application, First Publication No. 2000-35424
(Patent Document 3) Japanese Unexamined Patent Application, First Publication No. H11-330043